1. Field of the Invention
The present invention relates to an optical pickup and an optical information processing apparatus using the optical pickup by which a satisfactory beam spot on an information recording surface of a blue-system optical recording medium which uses a light source of a blue wavelength zone, to a DVD-system optical recording medium using a light source of a red wavelength zone, or to a CD-system optical recording medium using a light source of an infrared wavelength zone.
2. Description of the Background Art
Optical recording media, such as a CD with a storage capacity of 0.65 GB and a DVD with a storage capacity of 4.7 GB, are known as means to store image information data, voice information, or computer data. Further, demands for further improvement in recording density and storage capacity have become stronger in recent years. Specifically, the necessity for storage capacity such as 22 GB by which a high-definition television program can be stored for two hours for recording one movie program, or for 44 GB by which the same can be stored for four hours for recording a sports relay broadcast or the like.
In an optical pickup which performs information writing or informational reading to/from an optical recording medium, in order to improve the recording density of such an optical recording medium, it is effective to reduce the diameter of the beam spot. The beam spot may be formed by a beam condensed by an object lens and formed on an optical recording medium by enlarging the numerical aperture (which may be abbreviated as ‘NA’) of the object lens, or by shortening the wavelength of the light emitted by a light source. Therefore, for example, in a “DVD-system optical recording medium” with which high recording density is achievable for which the NA of the object lens is 0.65 and the light source emits 660 nm wavelength light in comparison to a “CD-system optical recording medium” for which the NA of the object lens is 0.50 and the light source emits 780 nm wavelength light.
As for such an optical recording medium, as mentioned above, further improvement in the recording density and storage capacity is demanded, and, for this purpose, it is desirable to increase the NA of the object lens (i.e., make the NA greater than 0.65), or to make the wavelength of the light source shorter than 660 nm.
There are several new standards for optical recording media. One known standard is the “HD-DVD” standard. It uses a light source with an operating wavelength of 405 nm and an object lens with an NA of 0.65 for an optical recording medium having a light-incidence-side substrate with a thickness of 0.6 mm. Another known standard is the “Blu-ray Disc” standard. The latter standard uses a light source with an operating wavelength of 405 nm and an object lens with an NA of 0.85 for an optical recording medium having a light-incidence-side substrate with a thickness of 0.1 mm.
Another problem is that users still have conventional recording medium such as CDs and DVDs. It is desirable that these conventional optical recording media and new optical recording media according to the above-mentioned new standard should be handled with a single common optical information processing device. A relatively easy method of solving this problem is that an optical pickup according to the conventional standard and an optical pickup according to the new standard are both mounted in one machine separately. However, this method may raise the cost or increase the size of the whole machine.
Thus, problems to be solved occurring when achieving an optical pickup directed to future high-density optical recording media are to reduce aberrations which otherwise increase due to increase in NA or reduction in wavelength, and, also, to achieve compatibility between the conventional standard and the new standard optical recording media (i.e., of different generations). Further, it is also a problem to be solved to achieve these objects without a remarkable increase in size/costs of the machine.
Furthermore, as described above, as long as the conventional recording media such as CDs and DVDs are applied, spherical aberration occurring in connection with a thickness error of the optical recording medium may be reduced by improving the manufacturing accuracy of the optical recording medium. Further, coma aberration in connection with an inclination of an optical recording medium may also be reduced by setting the substrate thickness at 0.1 mm, even when the light source of a blue wavelength zone and an object lens NA of 0.85 are applied.
However, in the future, in connection with achievement of high-speed rotation of an optical recording medium, application of a multi-layer-type optical recording medium, which will be described later, and also application of a multi-level recording scheme, it may not be possible to tolerate such aberration, and thus, a special scheme for correcting it should be needed.
For the above described problem, it is preferable to use three light sources for blue-system, DVD-system and CD-system optical recording medium with a single object lens for focusing an incident light beam from each light source. But there is a problem about aberration to focus for each blue-system, DVD-system and CD-system optical recording medium with a single object lens. For example, in the case of using a single object lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system (a parallel beam is incident to the object lens) and with a blue-system optical recording medium (λ1=405 nm, NA(λ1)=0.67, substrate thickness t1=0.6 mm), for focusing a beam spot on a DVD recording medium (λ2=660 nm, NA(λ2)=0.65, substrate thickness t2=0.6 mm), there arises a spherical aberration shown in FIG. 2B due to the difference in the wavelength. At this point, the horizontal axis of FIG. 2B represents the distance from the optical axis, and the vertical axis of FIG. 2B represents wavefront aberration.
As in the above described case, in the case of using a single object lens, which is designed for minimizing the wavefront aberration when it is used with an infinite optical system and with a blue-system optical recording medium (λ1=405 nm, NA(λ1)=0.67, substrate thickness t1=0.6 mm), for focusing a beam spot on a CD recording medium (λ3=785 nm, NA(λ3)=0.50, substrate thickness t3=1.2 mm), there arises a spherical aberration shown in FIG. 2C due to the difference in the wavelength and also due to the difference in the thickness. To reduce such a spherical aberration, it is known to use a finite optical system (a non-parallel beam is incident on the object lens) as described in Japanese Patent No. 3240846. Generally, changing the divergence angle of the beam that is incident on the object lens is the same as changing the spherical aberration. A beam divergence angle may be selected to reduce the spherical aberration. For example, the wavefront aberration can be suppressed when the object distance of the CD optical system (the distance between the optical source and the object lens) is changed.
FIG. 3B shows the relationship between the object distance and the wavefront aberration. It is indicative of deterioration at the wavefront surface. As shown in the drawings, the deterioration at the wavefront surface is decreased around 50 mm of object distance. Although FIG. 3 shows no parts between the object lens and the light source, actually there are a wavelength plate, a prism and a lens, etc. between the object lens and the light source. In particular, an optical pickup corresponding to three kinds of optical discs (blue-system, DVD-system and CD-system) has a lot of parts, so the above described 50 mm of object distance is too short. It is a limitation for parts alignment. And coma aberration occurs from object lens shifts with tracking movement or focusing movement in the finite system.
According to the background art, there is an optical pickup comprising a hologram allocated next to the object lens. The light beam for CD-system is transformed by the hologram from an infinite-system to a finite-system. It is described in Japanese Laid-open Patent No. 2003-294926.
However, because the hologram described in Japanese Laid-open Patent No. 2003-294926 is a polarization selectable hologram, it is impossible to use in a polarization light system. Recently the polarization light system is used in many optical information recording apparatuses, because the polarization light system can increase the efficiency of using a light beam. So it is preferable to use a polarization light system to an optical pickup corresponding to three kinds of wavelengths (blue-system, DVD-system and CD-system).